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Catalytic cracking and carbocation reaction mechanism Catalytic cracking is a process in which petroleum hydrocarbons are cracked at high temperatures in the presence of a catalyst to produce low-carbon olefins such as ethylene, propylene, butene, and light aromatics at the same time. Due to the presence of catalyst, catalytic cracking can reduce the reaction temperature, increase the yield of low-carbon olefins and light aromatic hydrocarbons, and improve the flexibility of cracked product distribution. (1) General characteristics of catalytic cracking ① Catalytic cracking is the result of the joint action of carbocation reaction mechanism and free radical reaction mechanism. The proportion of ethylene in the cracked gas product is greater than the proportion of ethylene in the catalytic cracking gas product. ② To a certain extent, catalytic cracking can be regarded as high-depth catalytic cracking. Its gas yield is much greater than that of catalytic cracking, and the aromatic hydrocarbon content in the liquid product is very high. ③ The reaction temperature of catalytic cracking is very high, and gas products with larger molecular weights will undergo secondary cracking reactions. In addition, low-carbon olefins will undergo hydrogen transfer reactions to generate alkanes, and polymerization or aromatization reactions will also occur to generate gasoline and diesel. (2) Reaction mechanism of catalytic cracking Generally speaking, both catalytic cracking reaction and thermal cracking reaction occur in the catalytic cracking process, which is the result of the joint action of carbocation and free radical reaction mechanisms. However, the specific cracking reaction mechanism varies with different catalysts and cracking processes. In the high-temperature cracking process over Ca-Al series catalysts, the free radical reaction mechanism dominates ； Carbocation reaction mechanism dominates during low-temperature cracking over acidic zeolite cracking catalysts ； In the medium-temperature cracking process on zeolite catalysts with dual acidic centers, both the carbocation mechanism and the free radical mechanism play an important role. (3) The influencing factors of catalytic cracking are similar to those of catalytic cracking. The factors affecting catalytic cracking also mainly include the following four aspects.: Raw material composition, catalyst properties, operating conditions and reaction equipment. ① Influence of the properties of raw oil. Generally speaking, the greater the H/C ratio and characteristic factor K of the feed oil, the higher the saturation content, and the lower the BMCI value, the higher the yield of low-carbon olefins (ethylene, propylene, butylene, etc.) obtained by cracking ； The greater the carbon residue value of the raw material and the higher the content of sulfur, nitrogen and heavy metals, the lower the yield of light olefins. When various groups of hydrocarbons are used as cracking raw materials, the order of yield of light olefins is generally: Alkanes > Naphthenes > Isoparaffins > Aromatic hydrocarbons. ② Catalyst properties. Catalytic cracking catalysts are divided into two types: metal oxide cracking catalysts and zeolite molecular sieve cracking catalysts. Catalyst is an important factor affecting product distribution in the catalytic cracking process. The cracking catalyst should have high activity and selectivity, not only to ensure that more low-carbon olefins are generated during the cracking process, but also to keep the yields of hydrogen, methane and liquid products as low as possible. It should also have high stability and mechanical strength. For zeolite molecular sieve type cracking catalysts, the pore structure, acidity and grain size of the molecular sieve are the three most important factors affecting the catalytic effect. ； For metal oxide cracking catalysts, the active components, carriers and additives of the catalyst are the most important factors affecting the catalytic effect. ③ Effect of operating conditions. The effect of operating conditions on catalytic cracking is similar to its effect on catalytic cracking. The better the atomization effect and gasification effect of the raw material, the higher the conversion rate of the raw material oil, and the higher the yield of low-carbon olefins. ； The higher the reaction temperature and the greater the agent-oil ratio, the higher the feed oil conversion rate and light olefin yield, but the coke yield also becomes larger. ； Since the reaction temperature of catalytic cracking is relatively high, in order to prevent excessive secondary reactions, the oil and gas residence time should not be too long. ； The impact of reaction pressure is relatively small. From a theoretical analysis, catalytic cracking should try to use high temperature, short residence time, large steam volume and large agent-to-oil ratio in order to achieve the maximum yield of low-carbon olefins. ④ The reactor is an important factor affecting the distribution of catalytic cracking products. The main types of reactors include fixed bed, moving bed, fluidized bed, riser and downward transport bed reactors. For the CPP process, the use of a pure riser reactor is beneficial to the production of more ethylene, and the use of a riser plus fluidized bed reactor is beneficial to the production of more propylene. (4) Introduction to catalytic cracking process Research on catalytic cracking of hydrocarbons has a history of half a century. Its research scope includes light hydrocarbons, distillate oils and heavy oils, and a variety of cracking processes have been developed. They are briefly introduced below. ① Catalytic cracking process (DCC process). This process was developed by the Sinopec Petrochemical Research Institute. It uses heavy oil as raw material, uses a solid acid shape-selective molecular sieve catalyst, and performs cracking reactions under mild reaction conditions to produce low-carbon olefins or isomeric olefins and high-octane gasoline. This process draws on fluidized catalytic cracking technology, adopts catalyst fluidization, continuous reaction and regeneration technology, and has achieved industrialization. The DCC process has two operating modes - DCC-I and DCC-II. DCC-Ⅰ adopts relatively harsh operating conditions and reacts in a riser dense-phase fluidized bed reactor to produce gaseous olefins mainly propylene in maximum quantities. ； DCC-Ⅱ adopts milder operating conditions and reacts in a riser reactor to produce the largest amount of small molecular olefins such as propylene, isobutylene and isopentene, and at the same time produce high-octane high-quality gasoline. ② Catalytic thermal cracking process (CPP process). This process is a patented technology for producing ethylene and propylene developed by Sinopec Petrochemical Research Institute. Based on traditional catalytic cracking technology, it uses heavy oils such as wax oil, wax oil mixed with residual oil, or atmospheric residual oil as raw materials. It uses a riser reactor and a specially developed catalyst and a continuous reaction-regeneration cycle operation mode of fluidized transportation of the catalyst to produce ethylene and propylene under operating conditions that are gentler than steam cracking. The CPP process is developed on the basis of the catalytic cracking DCC process. Its key technology is to convert its target product from propylene into ethylene and propylene through further improvement of the process and catalyst. ③ Direct cracking of heavy oil to produce ethylene (HCC process). This process was developed by the Refining Research Institute of Luoyang Petrochemical Engineering Company. It is a catalytic cracking process that directly cracks heavy oil to produce ethylene and also produces propylene, butene and light aromatics. It draws on the mature heavy oil catalytic cracking process, adopts fluidized "reaction-regeneration" technology, and uses riser reactors or downflow reactors to achieve high-temperature short-contact process requirements. ④ Other catalytic cracking processes. Such as catalytic-steam thermal cracking process (reaction temperature is generally very high, around 800°C), THR process (heavy oil catalytic conversion and catalytic cracking process developed by Toyo Engineering Company of Japan), rapid cracking technology (a set of catalytic cracking to produce olefins process jointly developed by Stone & Webster Company and Chevron Company), etc. ⑤ The cracking effect of paraffin-based raw materials is better than that of cycloalkyl-based raw materials. Therefore, most catalytic cracking processes use paraffin-based distillates or heavy oils as cracking raw materials. For naphthenic raw materials, especially distillates and hydrogenated distillates obtained from Canadian oil sand bitumen, heavy oils * * Professor Shen Baojian of the Key Laboratory has developed a specialized cracking catalyst. Preliminary evaluation results show that the total yield of ethylene and propylene is close to 30 wt%. (5) The difference between catalytic cracking and catalytic cracking. To a certain extent, catalytic cracking is developed on the basis of catalytic cracking, but there are obvious differences between the two, as follows: ① The purpose is different. Catalytic cracking aims to produce light oil products such as gasoline, kerosene and diesel, while catalytic cracking aims to produce basic chemical raw materials such as ethylene, propylene, butylene and butadiene. ② The raw materials are different. The raw materials for catalytic cracking are generally vacuum distillate oil, coked wax oil, atmospheric residual oil, and vacuum distillate oil mixed with vacuum residual oil. ； The range of raw materials for catalytic cracking is relatively wide, including catalytic cracking raw materials, naphtha, diesel, and C4 and C5 light hydrocarbons. ③ Catalysts are different. Catalysts for catalytic cracking are generally zeolite molecular sieve catalysts and aluminum silicate catalysts, while catalysts for catalytic cracking are generally zeolite molecular sieve catalysts and metal oxide catalysts. ④ Operating conditions vary. Compared with catalytic cracking, the reaction temperature of catalytic cracking is higher, the ratio of agent to oil is larger, the amount of steam is larger, the residence time of oil and gas is shorter, and the secondary reaction is more serious. ⑤ The reaction mechanism is different. The reaction mechanism of catalytic cracking is generally considered to be a carbocation mechanism, and the reaction mechanism of catalytic cracking includes both a carbocation mechanism and a free radical mechanism.